Superluminescent Halide Perovskite Light-Emitting Diodes with a Sub-Bandgap Turn-On Voltage

ABSTRACT

An emissive perovskite ternary composite thin film comprising a perovskite material, an ionic-conducting polymer and an ionic-insulating polymer is provided. Additionally, a single-layer LEDs is described using a composite thin film of organometal halide perovskite (Pero), an ionic-conducting polymer (ICP) and an ionic-insulating polymer (IIP). The LEDs with Pero-ICP-IIP composite thin films exhibit a low turn-on voltage of about 1.9V (defined at 1 cd m −2  luminance) and a luminance of about 600,000 cd m −2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 15/626,759filed on Jun. 19, 2017, which is a continuation-in-part of and claimspriority to International Application Serial No. PCT/US16/36386 filed onJun. 8, 2016, which claims priority to provisional U.S. Application Ser.No. 62/172,499 filed on Jun. 8, 2015. The present invention also claimspriority to U.S. Provisional Patent Application Ser. No. 62/351,323filed on Jun. 17, 2016. The related applications mentioned above arehereby incorporated by reference in their entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No.FA9550-16-1-0124 awarded by the U.S. Air Force Office of ScientificResearch, and Grant No. ECCS1609032 awarded by the National ScienceFoundation. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates, generally, to light-emitting diodes (LEDs). Morespecifically, it relates to single-layer LEDs utilizing a composite thinfilm of organometal halide perovskite polymer.

Brief Description of the Prior Art

Organometal halide perovskites (‘peros’) are materials with an ABX3crystal structure resembling that commonly found in barium titanate(BaTiO3). More specifically, in peros, A is a cesium (Cs+) or analiphatic ammonium (RNH3+) cation, B is a divalent cation of lead (Pb2+)or tin (Sn2+), and X is an anion such as chloride (Cl−), bromide (Br−),or iodide (I−). Peros have recently been discovered to have remarkableoptoelectronic properties, eliciting research into their potential asphotovoltaic and light-emitting devices

In recent years, LEDs have evolved as important commercial products toreplace traditional incandescent and fluorescent light bulbs for displayand lighting applications. The need for larger device size and lowerfabrication cost has motivated the exploration of novel LED technologiesincluding organic LEDs (OLEDs) based on organic small molecular andpolymeric semiconductors, quantum-dot LEDs, and more recently Pero-LEDs.Pero-LEDs may be made from a group of ABX₃ halide perovskitesemiconductors. For example, A may be a cesium (Cs⁺) or an aliphaticammonium (RNH³⁺) cation, B may be a divalent Pb²⁺ cation and X may be ahalide anion such as Cl⁻, Br⁻ or I⁻. Halide perovskites have shown theadvantages of low-temperature and cost-effective processing, and havealso manifested exceptional electronic and optical properties that aredesired for efficient electroluminescent devices.

Exemplary Pero-LEDs may be based on methylammonium lead halides(CH₃NH₃PbX₃, hereafter denoted as MA-Pero) or cesium lead halides(CsPbX₃, hereafter as Cs-Pero). Until now, the best MA-Pero LEDs thatemit green light (peak wavelength at 520-540 nm) had a maximum luminanceof about 20,000 cd m⁻², and the best Cs-Pero based green LEDs had amaximum luminance of 3,853 cd m⁻², both of which are much lower than thestate of the art in OLEDs, quantum-dot LEDs, and gallium nitride basedLEDs.

The relatively low luminance in Pero-LEDs can be attributed toinefficient electron and hole injection from the cathode and anoderespectively into the halide perovskite emitters. The majority ofreported Pero-LEDs use a multilayer device structure to enhance bothelectron and hole injection, and their turn-on voltages (defined at 1 cdm⁻² luminance) were found to be much higher than the bandgap (E_(g))/eof the perovskite emitters. For instance, a turn-on voltage of more than3.0 V was commonly reported among green Pero-LEDs that emitted photonswith an energy of about 2.3 eV. The only exception was observed by Wanget al. in their green Pero-LEDs which had a turn on voltage of 2.1 V anda device structure of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine)/molybdenum oxide as the hole injection layers (HILs) andpolyethyleneimine-modified zinc oxide as the electron injection layres(EILs). Nonetheless, the perovskite emissive layer in such a device haddiscontinuous coverage, and current leakage prevented it from reaching ahigh luminance intensity.

Accordingly, what is needed is a method for forming single-layer PeroLEDs having an improved luminance intensity. However, in view of the artconsidered as a whole at the time the present invention was made, it wasnot obvious to those of ordinary skill in the field of this inventionhow the shortcomings of the prior art could be overcome.

All referenced publications are incorporated herein by reference intheir entirety. Furthermore, where a definition or use of a term in areference, which is incorporated by reference herein, is inconsistent orcontrary to the definition of that term provided herein, the definitionof that term provided herein applies and the definition of that term inthe reference does not apply.

While certain aspects of conventional technologies have been discussedto facilitate the disclosure, Applicants in no way disclaim thesetechnical aspects, and it is contemplated that the disclosure mayencompass one or more of the conventional technical aspects discussedherein.

The present disclosure may address one or more of the problems anddeficiencies of the prior art discussed above. However, it iscontemplated that the disclosure may prove useful in addressing otherproblems and deficiencies in a number of technical areas. Therefore, thedisclosure should not necessarily be construed as limited to addressingany of the particular problems or deficiencies discussed herein.

In this specification, where a document, act or item of knowledge isreferred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date, publicly available, known to thepublic, part of common general knowledge, or otherwise constitutes priorart under the applicable statutory provisions; or is known to berelevant to an attempt to solve any problem with which thisspecification is concerned.

BRIEF SUMMARY OF THE INVENTION

The long-standing but heretofore unfulfilled need for an improvedperovskite ternary composite thin film, and a single-layer Perooptoelectronic device which utilizes the improved thin film, to provideimproved luminance intensity is now met by a new, useful, and nonobviousinvention.

In one embodiment, the present invention provides, an emissiveperovskite ternary composite thin film includes a perovskite material,an ionic-conducting polymer and an ionic-insulating polymer.

In various embodiments, the perovskite material may be an organometalhalide perovskite material and more specifically, the perovskitematerial may be a methylammonium lead halide (CH₃NH₃PbX₃).

In a particular embodiment, the perovskite material is selected fromcesium lead halide (CsPbX₃) and cesium lead tribromide (CsPbBr₃).

The ionic-conducting polymer may be PEO (poly(ethylene oxide)) and theionic-insulating polymer may be a PVP (poly(vinylpyrolidone)).

In a specific embodiment, the perovskite material is cesium leadtribromide (CsPbBr₃), the ionic conducting polymer is PEO (poly(ethyleneoxide)) and the ionic-insulating polymer is PVP (poly(vinylpyrolidone)).

In an additional embodiment, a single-layer thin film optoelectronicdevice is provided which includes, an anode, an emissive perovskiteternary composite thin film comprising, a perovskite material, anionic-conducting polymer and an ionic-insulating polymer and a cathode.

A method for manufacturing a single-layer optoelectronic device isadditionally provided, which includes, forming a perovskite material,adding an ionic-conducting polymer to the perovskite material to form aperovskite and ionic-conducting polymer mixture, adding anionic-insulating polymer to the perovskite and ionic-conducting polymermixture to form a perovskite ionic-conducting polymer andionic-insulating polymer mixture, coating a substrate with theperovskite ionic-conducting polymer and ionic-insulating polymer mixtureto form a thin-film layer on the substrate and annealing the thin-filmlayer.

These and other important objects, advantages, and features of theinvention will become clear as this disclosure proceeds.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of parts that will beexemplified in the disclosure set forth hereinafter and the scope of theinvention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1A is a top view SEM image of a thin film with CsPbBr₃:PEO=100:50.

FIG. 1B is a tilted view SEM image of the thin film of FIG. 1A.

FIG. 1C is a top-view SEM image of a thin film withCsPbBr₃:PEO:PVP=100:50:5.

FIG. 1D is a tilted view SEM image of the thin film of FIG. 1C.

FIG. 1E is a top-view SEM image of a thin film withCsPbBr₃:PEO:PVP=100:50:15.

FIG. 1F is a top-view SEM image of a thin film withCsPbBr₃:PEO:PVP=100:50:15.

FIG. 2 is a top-view SEM image of a thin film with CsPbBr₃:PEO=100:100.

FIG. 3A is a top view SEM image of a thin film with CsPbBr₃:PVP=100:5.

FIG. 3B is a cross-sectional tilted view SEM image of the thin film ofFIG. 3A.

FIG. 3C is a top view SEM image of a thin film with CsPbBr₃:PVP=100:30.

FIG. 3D is a cross-sectional tilted view SEM image of the thin film ofFIG. 3C.

FIG. 4A illustrates XRD patterns of CsPbBr₃:PEO (100:50) andCsPbBr₃:PEO:PVP (100:50:5) composite thin films.

FIG. 4B is a graph of photoluminescence and absorbance spectra ofCsPbBr₃:PEO:PVP (100:50:5) composite thin film.

FIG. 5A is a graph of current density versus voltage characteristics ofsingle-layer Cs-Pero LEDs with different PVP compositions in theemissive layers. The inset schematically illustrates the LED devicestructure as “ITO anode/CsPbBr3-polymer composite/In—Ga cathode”.

FIG. 5B is a graph of luminance versus voltage characteristics ofsingle-layer Cs-Pero LEDs with different PVP compositions in theemissive layers.

FIG. 5C is a graph of electroluminescence spectra collected at variousluminance intensities from a device with CsPbBr₃:PEO:PVP=100:50:5 in theemissive layer.

FIG. 5D shows photographs of lit devices according to the presentdisclosure operating at 2 V bias in dark (left) and at 4 V bias at anindoor lighting environment (right).

FIG. 5E is a graph of current efficiency/EQE versus voltagecharacteristics of single-layer Cs-Pero LEDs according to the presentdisclosure.

FIG. 5F is a comparison of maximum luminance and power efficiency ofCs-Pero LEDs according to the present disclosure with reported Cs-Peroand MA-Pero green LEDs.

FIG. 6A is a graph of normalized current density-voltage characteristicsof devices (CsPbBr₃:PEO:PVP=100:50:5) with emissive layer thickness of150 nm, 200 nm, and 250 nm.

FIG. 6B is a graph of normalized luminance-voltage characteristics ofdevices (CsPbBr₃:PEO:PVP=100:50:5) with emissive layer thickness of 150nm, 200 nm, and 250 nm.

FIG. 6C is an illustration of intervalley transfer in CsPbBr₃ leading toan NDR transition and declined emission characteristics in Cs-Pero LEDsas more electrons entering a higher energy satellite valley after acritical electric field.

FIG. 7A is a schematic illustration of junction formation in theCsPbBr₃-polymer composite film under an external electrical field.Cations and/or anions migrate toward and accumulate at theperovskite/cathode and perovskite/anode interfaces, resulting information of a p-i-n junction.

FIG. 7B is a graph of discharging current evolution with time byshort-circuiting a pre-biased LED device. The device had an emissivelayer thickness of 2 μm, and was exposed to 20 V for 10 seconds priorthe discharging test. The insets are enlarged portions showing the photoresponse of the device using a commercial white LED light source (˜10mW/cm²).

FIG. 7C is a band diagram schematic at the anode/CsPbBr₃ interface forthe single-layer LED device with (solid lines) and without (dot-dashedlines) junction formation, manifesting the narrowing of barrier widthafter ion migration and accumulation, which is equivalent to loweringthe effective barrier height. e is elementary charge, V represents thevoltage drop across the anode/CsPbBr₃ interface, and E_(F) is the quasiFermi level of CsPbBr₃ in the composite film.

FIG. 8 is a graph of photocurrent response of an un-biased Pero-LEDdevice.

FIG. 9 is a band diagram schematic at the cathode/composite interfacefor ITO/CsPbBr₃-polymer composite/In—Ga device with (solid lines) andwithout (dot-dashed lines) junction formation, manifesting the narrowingof barrier width after ion migration and accumulation. e is elementarycharge, V′ represents the voltage drop across the cathode/CsPbBr₃interface, and E_(F) is the quasi Fermi level of CsPbBr₃ in thecomposite film.

FIG. 10A is a graph of a stress test of Pero-LEDs with the 100:50:5emissive layer at 3.5 V bias in a nitrogen filled dry box.

FIG. 10B is a graph of a stress test of Pero-LEDs with the 100:50:5emissive layer at 2.7 V bias in a nitrogen filled dry box.

FIG. 11 is a photograph of one 100:50:5 device at 4 V under sunlight.The devices were illuminated in ambient air (30° C. and 60% relativehumidity).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a partthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

Charge-carrier injection into an emissive semiconductor thin film canresult in electroluminescence and is generally achieved by using amultilayer device structure, which requires an electron-injection layer(EIL) between the cathode and the emissive layer, and a hole-injectionlayer (HIL) between the anode and the emissive layer. The recentadvancement of halide perovskite semiconductors opens up a new path toelectroluminescent devices with a greatly simplified device structure.Various LEDs known in the art may comprise efficient charge-carrierinjection into the halide perovskite thin film without the aid of an EILor HIL. These so-called single-layer light-emitting diodes (LEDs) havebeen shown to exhibit a sub-bandgap turn-on voltage. The known devicesobtained a brightness of 591,197 cd m−2 at 4.8 V, with an externalquantum efficiency (EQE) of 5.7% and a power efficiency of 14.1 lm W−1.It was further discovered that intervalley carrier transfer in thehalide perovskite can be triggered after a threshold electrical field(˜25-31.2 MV m−1), which drastically reduced its radiative emissioncharacteristics, therefore limiting further brightness increase inhalide perovskite LEDs (Pero-LEDs).

In the present invention, various embodiments comprise superluminescentPero-LEDs with an exemplary composite emissive layer comprising cesiumlead tribromide (CsPbBr₃), poly(ethylene oxide) (PEO) andpoly(vinylpyrrolidinone) (PVP). The LEDs were constructed with an indiumtin oxide (ITO) anode, CsPbBr₃-PEO-PVP composite emissive layer, andindium-gallium eutectic (In—Ga) cathode without intentionally employingan EIL or HIL. Such single-layer devices start emitting green light at1.9 V and reach a maximum luminance of 593,178 cd m−2 at 4.9 V. Thesub-bandgap turn-on voltage suggests very efficient charge-carrierinjection, and it is hypothesize that a p-i-n junction may be formedin-situ in the perovskite emissive layer when an external bias isapplied. The efficient electron and hole injection eventually leads toan ultra-high brightness which is about 30 times that of the previousrecord in MA-Pero LEDs and 150 times that of Cs-Pero LEDs. Such anadvancement also makes Pero-LEDs of the present disclosure five timesbrighter than the vacuum evaporated OLEDs and three times as bright assolution processed quantum-dot LEDs.

The emissive perovskite composite thin films in single-layer Pero-LEDsaccording to various embodiments were obtained by spin coating a mixturesolution containing the CsPbBr₃ precursors, poly(ethylene oxide) (PEO)and poly(vinylpyrolidone) (PVP) with a desired weight ratio. It has beenshown direct spin-coating of the MA-Pero or Cs-Pero precursor solutiononto an ITO surface usually leads to a discontinuous film. The filmmorphology of MA-Pero was improved by blending it with an ionicconducting polymer (for example, but not limited to, PEO). Such acomposite film has been successfully applied as the emissive layer inPero-LEDs. However, mixing the CsPbBr₃ with PEO did not produce acontinuous film as shown in the scanning electron microscope (SEM) imagein FIG. 1A. The film with CsPbBr₃:PEO=100:50 ratio (by weight andhereafter) consisted of isolated crystals of about 0.5-2 μm in size. Theoverall surface coverage of the film was about 60%, leaving theremaining area nearly uncovered as seen from the cross-sectional SEMimage in FIG. 1B. Increasing the PEO content in the composite did notimprove film continuity, and caused the CsPbBr₃ crystals to becomefurther separated, reducing their total surface coverage to about 50% asshown in FIG. 2.

In contrast, it was found the CsPbBr₃ can be well dispersed by PVP. Acomposite film with CsPbBr₃:PVP=100:5 had a nearly continuous morphologyexcept for some scattered voids of about 20 nm diameter (FIGS. 3A and3B). The film became fully continuous and pinhole free atCsPbBr₃:PVP=100:30 (FIGS. 3C and 3D). Inspired by such an observation,PVP was then added to the CsPbBr₃/PEO composite. As shown in FIGS. 1Cand 1D from a film with CsPbBr₃:PEO:PVP=100:50:5, all the pinholespreviously seen in the CsPbBr₃/PEO composite had been removed and adense composite film was formed. At higher PVP ratios(CsPbBr₃:PEO:PVP=100:50:15 and 100:50:30) the films remained continuous,however, the density of the CsPbBr₃ crystals became reduced (FIGS. 1Eand 1F).

The crystallinity of the composite films were characterized by X-raydiffraction (XRD). The XRD patterns shown in FIG. 4A indicated both thebinary and the ternary composite films consisted of polycrystallineCsPbBr₃ belonging to an orthorhombic phase. The lattice parameters ofthe CsPbBr₃ were calculated to be a=0.821 nm, b=0.826 nm, and c=1.176nm, which are in agreement with literature values. Absorbance andphotoluminescence (PL) spectra were also collected to evaluate theoptical properties of the composite thin films. All films with variousPVP ratios behaved nearly the same, and the measurement results of aCsPbBr₃:PEO:PVP=100:50:5 film are presented in FIG. 4B. A sharptransition is observed at around ˜525 nm in the absorbance spectrum thatagrees well with the peak position in the PL spectrum, bothcorresponding to a bandgap of 2.36 eV in CsPbBr₃.

LEDs were constructed using an ITO anode/CsPbBr₃-polymer composite/In—Gacathode as illustrated in FIG. 5A inset. The active area of the deviceswas about 2 mm diameter as defined by the area of the In—Ga electrode.Current density-voltage (J-V) and luminance-voltage (L-V)characteristics were collected for devices with CsPbBr₃:PEO=100:50, andCsPbBr₃:PEO:PVP=100:50:5 and 100:50:15 in the emissive layersrespectively (FIGS. 5A and 5B). Noticeably, the device without PVP(CsPbBr₃:PEO=100:50) showed a current density about two orders ofmagnitude higher before 1.9 V when compared to the 100:50:5 and100:50:15 devices. This may be attributed to electrical leakage due tothe discontinuous nature of the binary composite film as shown in FIGS.1A and 1B. The current densities started to rapidly increase after 1.9 Vfor all the devices, reaching peak values of 2,226 mA cm⁻² at 4.3 V forthe 100:50 composite, 2,787 mA cm⁻² at 4.9 V for the 100:50:5 composite,and 1,226 mA cm⁻² at 4.7 V for the 100:50:15 composite. Immediatelyafter the peak values, all J-V curves entered a negative differentialresistance (NDR) region, which extended for an increment of 0.7-1 Vexternal bias. The current density rose up again after the NDRtransition, however with a more gradual slope than the onset at 1.9 V.

As illustrated by the L-V characteristics in FIG. 5B, the 100:50:5device turned-on at 1.9 V and the luminance increased rapidly withvoltage, reaching 100,000 cd m⁻² at 3.7 V and a maximum of 593,178 cdm⁻² at 4.9 V. The other devices both had a turn-on voltage at 2.2V,increasing to a maximum of 54,930 cd m⁻² at 4.5 V for the 100:50 deviceand 17,543 cd m⁻² at 4.4 V for the 100:50:15 device. All devices emittedgreen light, and FIG. 5C shows the EL spectra of the 100:50:5 device atvarious luminance intensities. All spectra had peak positions located at˜522 nm with a FWHM of 21 nm, correlating quite well to the PL spectrumin FIG. 4B. FIG.5D showed two optical images of a lit LED device with100:50:5 composite emissive film at 2 V (left) and 4 V (right) appliedbias. Real-time videos were also recorded for devices lit at 4 V and 2V. FIG. 5E shows the efficiency versus voltage characteristics of LEDswith 100:50, 100:50:5 and 100:50:15 composite emissive layers. Thecurrent efficiency achieved a maximum of 21.5 cd A⁻¹ at 4.8 V,corresponding to an EQE of 5.7% for the 100:50:5 device; 2.5 cd A⁻¹ at4.5 V corresponding to an EQE of 0.7% for the 100:50 device; and 1.6 cdA⁻¹ at 4.3 V corresponding to an EQE of 0.4% for the 100:50:15 device.

To evaluate the reproducibility of the best performance, eight deviceswere fabricated in a single batch with the 100:50:5 composite emissivelayer. The device performances are summarized in Table 1. The turn-onvoltage varied from 1.8 to 2.1 V, maximum current efficiency from 16.3to 25.6 cd A⁻¹, EQE from 4.3% to 6.8%, maximum power efficiency from 9.6to 14.9 lm W⁻¹, and maximum brightness from 416,744 to 804,719 cd m⁻².All devices exhibit very high luminance intensities and high powerefficiencies. It is worth noting that the combination of high luminousefficiency and high luminance of Pero-LEDs according to variousembodiments has greatly outperformed all reported devices using MA-Peroor Cs-Pero as the emissive layer (FIG. 5F).

TABLE 1 Summary of Performance for Eight Pero-LEDs in a Single Batchwith 100:50:5 Composite Emissive Layer Luminance Turn- Voltage at at onMaximum Maximum Maximum Maxi- Maximum De- Volt- Current Current Currentmum Power vice age Efficiency Efficiency Efficiency EQE Efficiency No.(V) (cd A⁻¹) (V) (cd m⁻²) (%) (lm W⁻¹) 1 2.0 16.3 5.3 416,744 4.3 9.6 21.8 25.6 5.4 453,306 6.8 14.9 3 2.0 22.4 5.7 498,953 6.0 12.4 4 2.1 20.45.3 530,423 5.4 12.1 5 2.0 20.3 6.0 586,395 5.4 10.6 6 1.9 21.5 4.8591,197 5.7 14.1 7 2.0 18.8 5.1 635,729 5.0 11.6 8 2.0 18.8 5.5 804,7195.0 10.7

To further understand the origin of the NDR phenomenon in the J-V curvesin FIG. 5A, devices (100:50:5) were fabricated with an emissive layerthickness of 150 nm, 200 nm, and 250 nm, respectively. As shown in FIG.6A, all devices exhibited a NDR transition with the threshold voltageoccurring at 3.8 V for the 150 nm device, 5.4 V for the 200 nm device,and 7.9 V for the 250 nm device, corresponding to an electrical field of25.3 MV m⁻¹, 27.0 MV m⁻¹, and 31.6 MV m⁻¹, respectively. Given theconsiderable thickness variation, the threshold electrical fieldappeared within a narrow range. The luminance of all the three devicesalso steeply declined after the threshold voltage (FIG. 6B). Based onboth observations, it is speculated that intervalley carrier transfermay have occurred in the CsPbBr₃/polymer composite film as schematicallyexplained in FIG. 6C: electrons start to transfer into a neighboringhigh-energy satellite valley above a threshold electrical field. Such atransfer may lead to a lower carrier mobility that contributes to thedeclining current within the NDR region and the more gradual slope ofcurrent increase after the NDR region. The intervalley transfer maygreatly decreases radiative recombination probability of the injectedcharge carriers, causing fast luminance decay after the thresholdvoltage.

Remarkably, the turn-on voltage in devices according to variousembodiments was 0.26-0.56 V lower than the E_(g)/e of the perovskiteemitter. Such an efficient turn-on is usually found in commercialinorganic LEDs that emit infrared, red or green light based on smallbandgap III-V semiconductors such as GaAs and AlGaInP. In those devices,both p and n type doping can be readily achieved and the employment of ap-i-n device structure effectively removes the charge injection barriersbetween the electrodes and the emissive semiconductor layer. Given theextremely simplified device structure in our work, it is hypothesizedthat a p-i-n junction may have formed in-situ within the CsPbBr₃-PEO-PVPcomposite film when an external bias was applied.

It has been reported that the methyl ammonium cations in MAPbI₃ canmigrate towards the cathode at a relatively low electrical field (<1 Vm⁻¹). Therefore, the ionic species in the CsPbBr₃-PEO-PVP composites mayrespond in a similar way as the methyl ammonium cations to an externalelectrical field, and develop net charges at the electrode/perovskiteinterfaces (FIG. 7A). The accumulation of ions can create a p-typeequivalent region along the anode/perovskite interface, and an n-typeequivalent region along the cathode/perovskite interface, thus form ananalogy of a p-i-n junction in the perovskite layer.

To verify the ion migration/accumulation hypothesis, time dependentdischarging current (I_(dis)) was measured as shown in FIG. 7B byshort-circuiting a pre-biased LED device. The I_(dis) started at 240 nA,and decayed exponentially with time. Such a behavior is in agreementwith the neutralization process of the accumulated ions through ionicdiffusion during the discharging test. Junction formation in theCsPbBr₃-PEO-PVP composite film is further supported by photo currentresponse (FIG. 7B insets) of a pre-biased device. A photo current of 150nA was measured under a light source irradiation within the first minuteof discharging, showing the separation of photo-induced carriers wasquite efficient due to the p-i-n junction formation. In contrast, thephoto current was ˜3 nA for a freshly prepared un-biased device (FIG.8), which is typical for a device without a p-i-n junction. After 15-16minutes of discharging, the photo current reduced to about 20 nA,suggesting the degradation of the junction during the dischargingprocess. As a result of large density ion accumulation and heavy doping,a sharp tunneling barrier evolves for electron and hole injection at theperovskite/electrode interfaces and greatly reduces contact resistancecompared to a conceptual device that doesn't form such a junction (FIGS.7C and 9). The in-situ junction formation has resulted in very efficientelectron and hole injection into the CsPbBr₃, which contributes to anultra-high luminance intensity in Pero-LEDs, according to variousembodiments.

The stability of devices according to various embodiments undercontinuous operation at a constant voltage was also evaluated at roomtemperature inside a nitrogen filled dry box with oxygen and moistureconcentrations both about 1 ppm. As shown in FIG. 10A, when the appliedbias was 3.5 V the EL intensity obtained a peak value of 33,680 cd m⁻²in 3 s; then decayed to about 13,000 cd m⁻² in one minute and remainedrelatively stable for more than six minutes. When 2.7 V was used, thedevice required about five minutes to reach its peak luminance (˜400 cdm⁻²), as shown in FIG. 10B, decaying to 50% of its peak value afterabout 50 minutes. The finite time for obtaining the peak luminance isconsistent with the above p-i-n junction hypothesis since the migrationof ionic species is required for the junction formation. In addition,the devices were illuminated in ambient air (30° C. and 60% relativehumidity) and showed a high brightness that was clearly seen undersunlight (FIG. 11). The stability of Pero-LEDs have seldom been reportedin literature, but devices according to various embodiments performedthe best among those reported.

In summary, in an exemplary embodiment of the present invention,pinhole-free CsPbBr₃-PEO-PVP ternary composite thin films were developedusing a one-step solution process. Single-layer LEDs were fabricatedwith a device structure of ITO/CsPbBr₃-PEO-PVP composite thinfilm/In—Ga. The LEDs of the present invention exhibited a sub-bandgapturn-on voltage of 1.9 V and an ultra-high luminance of 593,178 cd m⁻²with a maximum power efficiency of 14.1 lm W⁻¹. The low turn-on voltageand high luminance are both attributed to an in-situ junction formationin the perovskite composite thin film under an external bias.

EXPERIMENTAL

Materials: Lead(II) bromide (99.999%), cesium bromide (99.999%),N,N-dimethylformamide (DMF, anhydrous, 99.8%), dimethyl sulfoxide (DMSO,anhydrous, 99.9%), poly(ethylene oxide) (average M_(w) ˜5,000,000),poly(vinylpyrrolidinone) (average M_(w) ˜1,300,000), and indium-galliumeutectic (99.99%) were purchased from Sigma-Aldrich. All materials wereused as received.

Film preparation and characterizations: The Cs-Pero precursor solutionwas prepared by dissolving PbBr₂ and CsBr in a 1:1.5 molar ratio inanhydrous DMSO to give a concentration of 120 mg mL⁻¹. PEO and PVP weredissolved in DMF with a concentration of 10 mg mL⁻¹ and 50 mg mL⁻¹,respectively. The Pero precursor, PEO and PVP solutions were then mixedwith desired ratio. All the solutions were stirred at 120° C. for 30mins before use. The ITO/glass substrates (20 ohms sq⁻¹) were cleanedsubsequently with detergent water, deionized water, acetone andisopropanol for 5 mins with sonication, and then blow dried withnitrogen and treated with oxygen plasma at 100 W power for 2 mins. Themixture solution was spin-coated onto the ITO/glass at 1500 rpm for 1min. The films were then annealed at 200° C. for 30 seconds. Solutionand film preparation, and following device testing were carried outinside a nitrogen filled glove box with oxygen and moisture level bothat ˜1 ppm. Commercial tools of field emission SEM (Zeiss 1540 EsB) andUV-Vis-NIR spectrometer (Varian Cary 5000) were used to characterize thecomposite thin films. PL spectra were collected at room temperature on aHoriba Jobin Yvon FluoroMax-4 Fluorometer. The excitation wavelength wasfixed at 460 nm. The emission spectra from 480 to 780 nm were collectedwith an integration time of 0.1 s.

LED measurement: Current density-voltage and luminance-voltagecharacteristics were measured with a Keithley 2410 source meter and asilicon photodiode. The silicon photodiode was further calibrated by aPhoto Research PR-655 spectroradiometer. The EL spectra were collectedby the PR-655 with neutral density filters providing attenuation down to3% within visible wavelength region.

The advantages set forth above, and those made apparent from theforegoing description, are efficiently attained. Since certain changesmay be made in the above construction without departing from the scopeof the invention, it is intended that all matters contained in theforegoing description or shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention that, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A method for manufacturing a single-layeroptoelectronic device, the method comprising: forming a perovskitematerial; adding an ionic-conducting polymer to the perovskite materialto form a perovskite and ionic-conducting polymer mixture; adding anionic-insulating polymer to the perovskite and ionic-conducting polymermixture to form a perovskite ionic-conducting polymer andionic-insulating polymer mixture; coating a substrate with theperovskite ionic-conducting polymer and ionic-insulating polymer mixtureto form a thin-film layer on the substrate; and annealing the thin-filmlayer.
 2. The method of claim 1, wherein the perovskite material iscesium lead tribromide (CsPbBr₃), the ionic-conducting polymer is PEO(poly(ethylene oxide)), and the ionic-insulating polymer is PVP(poly(vinylpyrolidone)).
 3. The method of claim 1, wherein theperovskite material is an organometal halide perovskite material.
 4. Themethod of claim 1, wherein the perovskite material is a methylammoniumlead halide (CH₃NH₃PbX₃).
 5. The method of claim 1, wherein theperovskite material is selected from the group consisting of cesium leadhalide (CsPbX₃) and cesium lead tribromide (CsPbBr₃).
 6. The method ofclaim 1, wherein the ionic-conducting polymer is PEO (poly(ethyleneoxide)).
 7. The method of claim 1, wherein the ionic-insulating polymeris PVP (poly(vinylpyrolidone)).
 8. The method of claim 1, wherein theperovskite material comprises lead tribromide (PbBr₃) and cesium bromide(CsBr) in a solution of dimethylformamide or dimethylsulfoxide.
 9. Themethod of claim 1, wherein the perovskite material comprises leadtribromide (PbBr₃) and cesium bromide (CsBr) in a solution ofdimethylsulfoxide (DMSO) at a molar ratio of about 1:1.5.
 10. The methodof claim 1, wherein the ionic-conducting polymer is PEO, and theionic-insulting is PVP, wherein the PEO and the PVP are dissolved indimethylformamide (DMF).